1. Field of the Invention
The present invention relates to a radio frequency dielectric filter basically including a dielectric coaxial resonator, and more particularly to a dielectric resonator having a simple construction including a single dielectric block and electrodes formed on the surface of the dielectric block, and a filter employing such a dielectric resonator.
2. Description of the Prior Art
Generally, UHF-band dielectric filters are mainly employed in RF transmitting and receiving units of portable telephones today. Such dielectric filters use 1/4 wavelength coaxial lines of TEM mode using a microwave dielectric exhibiting a high permittivity to achieve a compact microwave component.
It is desirable for terminal units of communication systems to be compact, which results in strong requirements for reducing the dimensions of the components of the terminal units.
In order to meet such requirements, there have been proposed various types of radio frequency filters using a dielectric. These conventional filters will now be described in conjunction with FIGS. 1 and 2.
FIG. 1 is a perspective view of a conventional filter using a dielectric. This filter includes unit dielectric resonators 101 coupled by capacitors. Coupling capacitors 106, each disposed between adjacent resonators 101, and capacitors 107, respectively constituting input and output capacitors of the filter, are constructed by electrodes 104 formed on a circuit board 105. The electrodes 104 on the circuit board 105 are connected to inner conductors 103, in holes which emerge at open surfaces 102 of the coaxial resonators 101, by means of conduction wires 108, respectively.
FIG. 2 is a perspective view of another conventional dielectric filter. This dielectric filter is a monoblock type dielectric filter including a single dielectric block. As shown in FIG. 2, the dielectric filter includes a dielectric block 201 and a plurality of holes 202 for forming coaxial resonators. By this construction including throughout holes 202, the distance between adjacent resonators is reduced, thereby achieving a compactness. In this case, holes 203 for attenuating the coupling degree have respective inner walls with no plated film. The input and output of the dielectric filter are constituted by capacitors which are constructed by inserting dielectric cylinders 204 made of a dielectric such as Teflon into resonators formed at opposite side portions of the filter, respectively. Each of the dielectric cylinders has a conduction rod 205 at its center portion.
However, the conventional dielectric filter construction of FIG. 1 has the disadvantages of a large volume and a complicated manufacturing process because the capacitors for coupling the resonators are constructed by forming electrode patterns 104 on the circuit board 105 separately provided, because it requires the conduction wires 108 for connecting the electrodes 104 to the inner conductors 103 of coaxial resonators, and because it also requires a package such as a metal case for providing a mechanical coupling between each resonator and the circuit board and providing input and output terminals.
Similarly, the dielectric filter construction of FIG. 2 has the disadvantages of a large volume and a complicated manufacturing process because each hole 203 for coupling degree attenuation should have an inner surface having no plated film, because the input and output capacitors are separately constructed by inserting the dielectric cylinders 204 with conduction rods 205 into resonators formed at opposite side portions of the filter, and because it also requires a package for providing the mechanical coupling.